THE ORIENTATION OF THE TRANSITION DIPOLE-MOMENTS OF CHLOROPHYLL-A ANDPHEOPHYTIN A IN THEIR MOLECULAR FRAME

In this paper we describe the determination of the orientation of the absorption and emission transition dipoles of chlorophyll a and pheoph ytin a in their molecular frame. For this purpose we have embedded the pigments in anhydrous nitrocellulose films with a concentration of 2 x 10(-7) mol/g. We have shown previously that under these conditions t he pigments are in a purely monomeric state, are distributed uniformly both before and after stretching and that no intermolecular energy tr ansfer among the molecules takes place. Using a combination of steady- state anisotropy experiments on unstretched films and angle-resolved f luorescence depolarization measurements on stretched films, we obtain the orientation of the transition dipole moments of both pigments in t heir molecular frame and the orientational distribution function of th e molecules relative to the stretching direction of the film. The stea dy-state anisotropy measurements indicate that chlorophyll a has two d istinct emission dipole moments and that excitation in the Soret-regio n results in simultaneous excitation of two or more absorption transit ion dipole moments. On the other hand, excitation in the Q(Y)-band inv olves only a single dipole moment. The directions of the transition di pole moments in the molecular frame are obtained from the angle-resolv ed measurements. Pheophytin a also exhibits two emission dipole moment s, but the angle between them is much smaller than that between the co rresponding dipoles for chlorophyll a. As a consequence the dipole mom ents contributing to the Soret-region could not be resolved and only a n effective absorption transition dipole moment in the Soret-region is extracted.